This invention relates to a LAN-to-LAN communication method as well as to a unit for interconnecting LANs in which LAN-to-LAN communication is performed in accordance with the status of each LAN terminal. Further, the invention relates to a LAN-to-LAN communication method, and a LAN-TO LAN connecting unit, in which LAN-to-LAN communication is performed by changing over a destination LAN-to-LAN connecting unit.
In order to interconnect LANs (local area networks) that are remote from each other, a so-called wide area network (WAN) such as a telephone network, data switching network, ISDN or leased circuit is used as the intermediary. A LAN-to-LAN connecting unit which is a router or bridge is provided between each LAN and the WAN.
A router has a LAN interface and a WAN interface. The LAN interface is connected to the LAN and controls first, second and third layers, namely a physical layer, a link layer and a network layer. The WAN interface is connected to the WAN and controls interfacing with the WAN. Communication between LANs using such routers is performed as follows: When a terminal sends out a frame having a MAC address and a network address attached thereto, the router executes filtering processing and accepts the necessary frame, refers to the network address contained in the frame and issues an outgoing call to the WAN to effect a connection to the party that is the destination of the call. The router then sends the frame to the LAN of the other party via the WAN. The router connected to the LAN of the other party accepts the received frame and sends it to this LAN. The LAN terminal having a MAC address identical with terminating-party MAC address contained in the frame accepts this frame.
The bridge has a LAN interface, a WAN interface and an office data table indicating the correlation between destination MAC addresses and telephone numbers. The LAN interface is connected to the LAN and the WAN interface is connected to the WAN. Communication between LANs using such bridges is performed as follows: When a terminal sends out a frame having a MAC address attached thereto, the bridge executes filtering processing, accepts the necessary frame and extracts the MAC address. Next, the bridge refers to the local data table to obtain the destination telephone number and issues an outgoing call using this telephone number to effect a connection to the party that is the destination of the call. Alternatively, the bridge can effect the connection to the destination manually in advance by issuing an outgoing call to the destination telephone number corresponding to the destination MAC address. Thereafter, the bridge sends the frame to the LAN of the other party via the WAN. The bridge connected to the LAN of the other party accepts the frame and sends it to this LAN. The LAN terminal having a MAC address identical with terminating-party MAC address contained in the frame accepts this frame.
FIG. 21 is a diagram illustrating the configuration of a LAN-to-LAN communication system. The system includes LANs (LAN 1, LAN 2) 1a, 1b such as Ethernets, LAN terminals 2a, 2b, a wide-area network (WAN) 3 such as a telephone network, data switching network, ISDN or leased circuit, and bridges or routers (LAN-to-LAN connecting units) 4a, 4b. The terminal 2a is assigned a MAC address 100, and the terminal 2b is assigned a MAC address 200.
FIG. 22 is a block diagram illustrating the LAN-to-LAN connecting unit (e.g., a router). The unit includes a LAN interface 5a connected to the LAN, a WAN interface 5b connected to the WAN, a sending/receiving controller 5c on the LAN side, an address data base 5d storing the MAC addresses of LAN terminals covered by the system, a processor 5e, a memory 5f, a sending/receiving controller 5g on the WAN side and an originating/terminating processor 5h.
FIG. 23 is a diagram showing the basic constitution of a frame. A set of data delimited by a start delimiter (SD) and an end delimiter (ED) is a frame. In a LAN, a frame is one unit of transmission. The frame starts with a start delimiter of one octet, which is followed by a control field of one or two octets. Depending upon the LAN, there are frames which do not have a control field. The control field is followed by a destination address (DA) and a source address (SA), then by an information field (I) for introducing the information desired to actually be transmitted, and a frame check sequence (FCS). In layer 2 (L2), DA, SA are MAC addresses and are allocated to each terminal. The above-mentioned MAC addresses are used in communication in which a WAN does not take part. However, in order to perform LAN-LAN communication via a WAN, the information field I is further subdivided into a control information field, a destination address DA, which is a network address (corresponding to a telephone number), a source address SA and an information field. The information field can be sent in the form of packets, in which case frames are referred to as packets.
When a transmission packet shown in FIG. 23 is issued by the LAN terminal 2a (FIG. 21), the LAN interface 5a of the router 4a accepts the packet and applies it to the sending/receiving controller 5c. The latter checks the address data base 5d to determine whether a MAC destination address DA is present in the data base. If the address is absent, the controller 5c inputs the received packet to the processor 5e. If the address exists, on the other hand, this means that the packet is that of another LAN terminal destination under the router's command. The controller 5c therefore discards the received packet. Further, the sending/receiving controller 5c checks the address data base 5d to determine whether a source address SA is present. If the address is absent, the controller 5c registers the source address SA in the address data base 5d.
The processor 5e buffers the packet from the send/receive controller 5c in the memory 5f, obtains the destination address DA serving as the network address and notifies the originating/terminating processor 5h. Upon being notified of the destination address DA (telephone number), the originating/terminating processor 5h sends an outgoing call to the WAN via the WAN interface 5b and establishes a connection with the LAN-to-LAN connecting unit of the other party. When the above-described call processing control ends, the sending/receiving controller 5g on the WAN side reads packets out of the memory 5f in succession and transmits these packets to the WAN via the WAN interface 5b. As a result, the transmitted packets are accepted by the LAN-to-LAN connecting unit on the terminating side.
The sending/receiving controller 5g of the LAN-to-LAN connecting unit on the terminating side stores the packets accepted via the WAN interface 5b, and the sending/receiving controller 5c on the LAN side reads the received packets out of the memory and transmits them to the LAN via the LAN interface 5a. The LAN terminal having a MAC address identical with the terminating MAC address contained in the packets accepts the packets and sends a response packet back to the LAN-to-LAN connecting unit on the terminating side.
The foregoing is an overview of operation in a case where LANs communicate via a WAN.
As shown in FIG. 24, there are cases in which the LAN terminal 2b on the receiving side is incapable of receiving data because a cable has been pulled out or as the result of some other problem. In such case the LAN-to-LAN connecting unit 4b on the terminating side sends a received packet to the LAN 2 and then waits for reception of a response packet. If the response packet is not received within a fixed period of time, then the connecting unit 4b disconnects the line. Thus, in conventional LAN-LAN communication, the line is eventually disconnected if the terminal on the terminating side is incapable of reception. However, since the line remains connected for the period of time during which reception of the response packet is being awaited, this waiting time also is billed. Thus, a problem with the conventional system is higher line cost.
Further, when the called party is incapable of reception in conventional LAN-LAN communication, it is necessary to perform the transmitting operation again to transmit data after the line has been disconnected. There are many cases in which it would be convenient if the data could be transferred to another terminal even if the receiving party is in a state in which reception is not possible. For example, if the receiving party is a printer terminal for printing received data and there is a printer terminal available separate from the former terminal, it would be convenient if the latter terminal could receive and print the data in the event that the former terminal is incapable of reception. With the conventional communication system, however, the data cannot be transferred to another terminal.